Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescence element and an organic electroluminescence element (hereinafter, referred to as an organic EL element).
An inorganic electroluminescence element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescence element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence and phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescence element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
However, in an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption.
In Japanese Patent No. 3093796, a slight amount of a fluorescent substance has been doped in a stilbene derivative, a distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element.
Further, there are known such as an element, having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped, with a slight amount of a fluorescent substance (for example, JP-A No. 63-264692) and an element having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, JP-A No. 3-255190).
In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is 1/3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of an external quantum efficiency (ηext) of taking out light is said to be 5% at maximum.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (M. A. Baldo et. al., Nature vol. 395, pp. 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active.
For example, it is also disclosed in A. Baldo et al. Nature, vol. 403, No. 17, pp. 750-753 (2000), and U.S. Pat. No. 6,097,147.
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet which is principally 4 times of the ease of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, in S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), many dopants mainly belonging to transition metal complexes such as iridium complexes and platinum complexes have been investigated.
Further, in the aforesaid, A. Baldo et al. Nature, vol. 403, No. 17, pp. 750-753 (2000), utilization of tris(2-phenylpyridine)iridium as a dopant (it is also called as a light emitting material) has been studied.
In addition to these, M. E. Tompson et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied to utilize L2Ir(acac) such as (ppy)2Ir(acac) as a dopant, Moon-Jae Youn. Og., Tetsuo Tsutsui et al., also at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied utilization of such as tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3) and tris(benzo[h]quinoline)iridium (Ir(bzq)3) (herein, these metal complexes are generally referred to as orthometalated iridium complexes.).
Further, in the aforesaid, S. Lamansky et al., J. Am. Chem. See, vol. 123, p. 4304 (2001), or in JP-A No. 2001-247859, studies have been carried out to prepare an element utilizing various types of iridium complexes.
As will be mentioned later, these complexes are used by being dispersed and added with a light emitting host material (or simply, it is called as a host) in a light emitting layer. And these complexes are called as a phosphorescent dopant.
Since the properties (light emitting efficiency, light emitting lifetime and light emission color) of an organic EL element will be largely changed not only by the dopant but by the host, the development of the dopant and the host have been extensively carried out.
For example, in order to obtain high emission efficiency, Ikai et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu) utilized a hole transporting compound as a host of a phosphorescent compound. Further, M. E. Tompson et al. utilized various types of electron transporting materials as a host of a phosphorescent compound doped with a new iridium complex.
In any cases, by suitably selecting a dopant and a host, emission luminance, and emission efficiency are significantly improved compared to conventional elements because the emitting light arises from phosphorescence. However, there has been a problem of a poor emission lifetime of the element compared to conventional phosphorescence type elements.
In particular, in the case of a blue light emitting element, she luminescence lifetime becomes extremely short, and the blue dopant which satisfied all requirements of luminescence efficiency, luminescence wavelength, and luminescence lifetime has not yet been found out, and the creation of such dopant is an urgent need.
As a blue phosphorescent dopant, it is known to introduce an electro negative group as a substituent in a phenyl pyridine group. Examples of such group are: a fluorine atom, a trifluoromethyl group and a cyano group. And it is also known to introduce picolinic acid or a pyrazabole system as a ligand.
However, by the complexes which use these ligands, it can be realized a blue luminescence by achieving shortening of luminescence wavelength, and it can achieve efficient luminescence. On the other hand, a luminescence lifetime of an element was very short. Improvement of trade-off of shortening of luminescence wavelength and luminescence lifetime has been required.
On the other hand, as a new development, it is known feat a metal complex which has a phenylpyrazole ligand can apply for producing blue phosphorescent luminescence.
However, it is known that an iridium complex having a simple pyrazole ligand does not emit light at room temperature, but emits light only after introducing a group which reduces a band gap in a substituent (refer to Patent document 6, for example).
However, this method did not improve the above-mentioned trade-off. After all, in order to improve luminescence property and luminescence lifetime, it is required to extend π-conjugated system to result in a long luminescence wavelength. Consequently, this method did not satisfy the requirements for blue phosphorescent dopant.
It is disclosed that a metal complex containing a phenylimidazole ligand is a light emitting material having a comparatively shortwave luminescence wavelength (refer to Patent documents 2 and 3, for example).
However, there remains a large problem at a present time when further shortening of wavelength and extension of lifetime are requested.
It is disclosed that a metal complex containing a phenylimidazole ligand has a comparatively short luminescence wavelength luminescent material (refer to Patent documents 4 and 5, for example).
However, there remains a large problem at a present time when low electric power consumption and extension of lifetime are requested.
Thus, it is the most difficult work for a phosphorescent dopant to simultaneously satisfy the requirements of (a) high emission property; (b) emission at short wavelength; and (c) extension of lifetime (it is also called high durability).
Among the efforts, it was reported that it was principally efficient to increase the transition probability from a metal portion, to a ligand portion in a metal complex with respect to high emission property (to enhance “Metal-to-ligand charge transfer (MLCT)”) (refer to Non-patent documents 1 and 2, for example).
As one of the enhancing means of MLCT property, there was carried out a study to incorporate a substituent having a vacant orbital such as an arylboryl group in a ligand (refer to Non-patent document 3, for example).
In this document, an absorption derived from MLCT and light emission cased by photoexcitation were observe by introducing a triaryl boryl group in a ter-pyridine-Pt complex, which usually does not emit light even with photoexcitation.
Moreover, apart from the different viewpoint from this, it was disclosed that an element achieving high color purity due to the inhibition of broadening of emission wave shape with highly effective was obtained by introducing a boron containing substituent in a phenyl pyridine ligand (refer to Patent document 1).
However, the above-mentioned patent documents mainly aimed at achieving longer wavelength of emission. Our investigation revealed that it cannot realize blue emission by using these metal complexes, and emission lifetime cannot be extended. They are the problems to be solved for future element development.